A. Field of the Invention
The present invention is related to a method for removing organic contaminants from a semiconductor surface.
The present invention is also related to the use of this method for specific applications such as cleaning steps after VIA etching and other etch processes.
B. Description of Related Art
The semiconductor surface preparation prior to various processing steps such as oxidation, deposition or growth processes, has become one of the most critical issues in semiconductor technology. With the rapid approach of sub halfmicron design rules, very small particles and low levels of contamination or material impurities (xcx9c1010 atoms/cm2 and lower) can have a drastic effect on process yields. The contaminants that are to be removed from a semiconductor surface include metallic impurities, particles and organic material. A commonly used technique to reduce foreign particulate matter contamination level on semiconductor surfaces is the immersion of wafers in chemical solutions.
Organic material is one of the contaminants that has to be removed from the semiconductor wafer surface. In a pre-clean stage, absorbed organic molecules prevent cleaning chemicals from contacting with the wafer surface, thus leading to non-uniform etching and cleaning on the wafer surface. In order to realize contamination free wafer surfaces, organic impurities have to be removed before other wafer cleaning processes. Traditional wet cleaning processes involve the use of sulfuric peroxide mixtures (SPM) to remove organic molecules. However, SPM uses expensive chemicals and requires high processing temperatures, and causes problems in terms of chemical waste treatment.
Other sources of organic contamination also arise during a standard IC process flow. Such sources can be photoresist layers or fluorocarbon polymer residues that are deposited on a substrate.
The fluorocarbon residues originate from the exposure of semiconductor (silicon) substrates to dry oxide etch chemistries. In conventional oxide etching with fluorocarbon gases, an amount of polymer is intentionally generated in order to achieve a vertical sidewall profile and better etch selectivity to the photoresist mask and underlying film. Etch selectivity in a SiO2xe2x80x94Si system can be achieved under certain process conditions through the formation of fluorocarbon based polymers. The polymerisation reaction occurs preferably on Si, thus forming a protective coating and etch selectivity between Si and SiO2. After selective etching, both resist and polymer-like residue must be removed from the surface. If the polymer is not completely removed prior to the subsequent metal deposition, the polymer will mix with sputtered metal atoms to form a high resistance material resulting in reliability concerns. Methods of polymer removal depend on the plasma etch chemistry, plasma source and the composition of the film stack. However, for dry processes, the application of O2 or H2 containing gases have been applied to remove the fluorocarbon polymers. For wet cleaning techniques an amine based solvent (U.S. Pat. No. 5,279,771 and U.S. Pat. No. 5,308,745, which are hereby incorporated by reference) is frequently applied. Organic photoresist removal generally involves wet or dry oxidative chemistries (i.e. O2 plasma, SPM) or dissolution processes based on solvent strippers. These processes are both expensive and environmentally harmful in terms of waste treatment.
In an attempt to find alternative efficient cleans for the removal of organic contamination (including photoresist and etch residues) from Si surfaces, the use of ozonated chemistries has been investigated. Ozone has been used extensively in the field of waste water treatment and drinking water sterilisation, because of its strong oxidising power. An additional benefit of ozone is its harmless residue after decomposition and/or reaction (H2O, CO2, O2). It is generally presumed that oxidative action of ozone towards organic contamination involves two different oxidation pathways, either direct oxidation or advanced oxidation. Direct oxidation or ozonolysis involves molecular ozone as the prime oxidant. It predominantly occurs at carbon-carbon double bonds. This type of oxidation is favored in the low pH region of the waste water. Advanced oxidation involves secondary oxidants as the prime oxidant (e.g. OH radicals). This type of oxidation is more reactive, but less sensitive and is predominant at conditions that favor OH radical formation, such as high pH, elevated temperature, addition of enhancers (e.g. H2O2), UV radiation. In real life situations, one often deals with a mixture of contaminants having a different reactivity towards ozone. However, both oxidation pathways are concurrent and conditions that favor advanced oxidation pathways will occur at the expense of the efficiency of eliminating organic contamination with higher reactivity towards molecular ozone. In order to optimize the organic removal efficiency of ozonated chemistries, it is critical to identify the parameters that influence both oxidation pathways.
In recent years, ozone was introduced in the microelectronics industry because of its strong oxidizing capabilities. When ozone gas is dissolved into water, its self-decomposition time gets shorter compared to the gaseous phase. During self-decomposition, ozone generates OH radicals as a reaction by-product, which is according to G. Alder and R. Hill in J. Am. Chem. Soc. 1950, 72 (1984), hereby incorporated by reference, believed to be the reason for decomposition of organic material.
U.S. Pat. No. 5,464,480, which is hereby incorporated by reference, describes a process for removing organic material from semi-conductor wafers. The wafers are contacted with a solution of ozone and water at a temperature between 1xc2x0 and 15xc2x0 C. Wafers are placed into a tank containing deionized water, while diffusing ozone into the (sub-ambient) deionized water for a time sufficient to oxidize the organic material from the wafer, while maintaining the deionized water at a temperature of about 1xc2x0 to about 15xc2x0 C., and thereafter rinsing the wafers with deionized water. The purpose of lowering the temperature of the solution to a range between 1xc2x0 and 15xc2x0 C. is to enable sufficiently high ozone concentrations into water to oxidize all of the organic material onto the wafer into insoluble gases.
European Patent Application EP-A-0548596 describes a spray-tool process, whereby during the cleaning process, various liquid chemicals, ultra-pure water or a mixed phase fluid comprising an ozone-containing gas and ultra pure water are sprayed onto substrates or semiconductor wafers in a treating chamber filled with ozone gas. Rotation is necessary to constantly renew thin films of treating solution and promoting removal of undesired materials by means of centrifugal force. It is concluded in this application that heating the substrate does not permit ozone to have well enough effects.
U.S. Pat. No. 5,181,985, which is hereby incorporated by reference, describes a process for the wet-chemical surface treatment of semiconductor wafers in which aqueous phases containing one or more chemically active substances in solution act on the wafer surface, with water in a finely divided liquid state such as a mist. The process consists of spraying the water mist over the wafer surface and then introducing chemically active substance in the gaseous state so that these gaseous substances are combined with the water mist in order to have an interaction of the gas phase and the liquid phase taking place on the surface of the semiconductor wafers. The chemical active substance are selected from the group consisting of gases of ammonia, hydrogen chloride, hydrogen fluoride, ozone, ozonized oxygen, chlorine and bromine. The water is introduced into the system at a temperature of 10xc2x0 C. to 90xc2x0 C.
U.S. Pat. No. 5,503,708, which is hereby incorporated by reference, describes a method and an apparatus for removing an organic film wherein a mixed gas including an alcohol and one of ozone gas and an ozone-containing gas is supplied into the processing chamber at least for a period before that the semiconductor wafer is placed in said processing chamber, so that the mixed gas will act on the organic film formed on the surface of the semiconductor wafer.
The present invention aims to suggest an improved method for the removal of organic contaminants from a semiconductor substrate.
More particularly, the present invention aims to suggest a method of removal of organic contamination such as photoresist, photoresidue, dry etched residue which can occur in any process step of the fabrication of semiconductor substrate.
Another aim of the invention is the removal of organic contamination from substrates, e.g. photoresist and photoresist residues, dry etch residues, from substrates during any process step of fabrication. The invention can be used in the fabrication of silicon wafers for Integrated Circuits. The invention can also be used in related fields, like the fabrication of flat panel displays, solar cells, or in micro-machining applications or in other fields wherein organic contaminants have to be removed from substrates.
A method and apparatus for removing organic contaminants from a substrate is disclosed.
The method comprises the steps of holding said substrate in a tank or reaction chamber, and filling said tank or chamber with a first gas mixture comprising water vapour and ozone. Preferably said substrate is thereafter rinsed with a fluid. The term tank for the purpose of this patent application is meant to cover any kind of tool or reaction chamber wherein substrates are held for the purpose of cleaning or removing organic contaminants. Thus the term tank is to cover tools or reaction chambers known in the art such as wet benches, vessels, spray processors, spinning tools, and single wafer cleaning tools.
The method is to be executed at a temperature higher than room temperature. The temperature can be in the range of 30-90xc2x0 C., preferably 50 or 80xc2x0 C., or higher. Thus the water can be a water vapour. It can also be water in the actual gas phase. A vapour is defined as the the gas phase occurrence of an element or a compound or a mixture of elements if the element or compound or mixture should be in the liquid or solid phase at the given temperature and pressure conditions. Thus a vapour can co-exist in one environment with the solid or liquid phase of the element. A vapour therefore also is not a mist of finely dispersed liquid droplets of an element or a compound or a mixture of elements. A vapour is a specific gas phase occurrence of an element or a compound or a mixture of elements. The method of the invention can comprise the steps of holding said substrate in a tank, said substrate being at a temperature above room temperature, filling said tank with a gas mixture comprising water and ozone in the gaseous phase; and optionally thereafter rinsing said substrate with a fluid. The method can also comprise the steps of filling said tank with water, the water level in said tank remaining below said substrate; heating the water thereby creating a water gas or a water vapor in said tank while filling said tank with ozone. The ozone can possibly be bubbled through the water. The rinse fluid can comprise water, the water level being elevated to have said substrate immersed in said fluid. Said fluid can further comprise HCl and/or HF and/or HNO3 and/or CO2 and/or H2SO4. Preferably the fluid is at 90xc2x0 C. and comprises 5% H2SO4 in H2O2. The fluid can also be subjected to megasone agitation.
The apparatus for removing organic contaminants from a substrate comprises a tank with a first inlet for ozone gas and a second inlet for a fluid, said second inlet being adapted for filling said tank to a plurality of predetermined fluid levels, said first inlet being adapted for diffusing said ozone gas, possibly through said fluid in said tank. The tank further comprises a substrate holder for holding said substrate in said tank, at least one of said predetermined fluid levels being under said substrate and means for heating said fluid above room temperature for creating a fluid vapor at a predetermined concentration level in said tank. The tank or chamber can also comprise means for heating said wafer. The ozone gas can be led through a bubbler or diffuser in the bottom of a tank, holding the substrate or a plurality of substrates. Note that the tank can be used to hold a cassette with substrates or to hold the substrates cassetteless. Water is fed into the tank through the water inlet. In a preferred embodiment, the water can be circulated through a tube with the opening above the ozone diffuser. In this way the ozone gas will be led through the water. By recirculating the water over a heater or a heat exchanger, the temperature of the water may be regulated and this will regulate the amount of water vapour inside the system. In another embodiment, the walls of the tank may be heated to regulate also the temperature of the substrates. This can be done for instance by means of IR or lamp heating, or using heated ozone. After the first process with water vapour and ozone is finished, rinse liquid will be filled inside the tank until it overflows. Then an option could be to recirculate this liquid several times over the substrates or the liquid may be drained immediately. The liquid preferably comprises water. The liquid can be heated and further comprise additives such as HF, HCl, HNO3 or CO2 or H2SO4.